1. Field of the Invention
The invention relates generally to data encoding. More particularly, the invention relates to high speed differential encoders.
2. Description of Related Art
Differential encoders produce an encoded data stream characterized by a change in the output state (from logical 1 to logical 0 or vice-versa) when an input logical 1 is present and no change when an input logical 0 is present. Thus, a differential encoder continually outputs the Boolean Exclusive-Or (XOR) of the previous output bit with the current input bit.
These differential encoders have latency problems that affect the data rates achievable. Typically, feedback around a flip-flop is used to compare the present input bit with the previous output bit. The feedback loop has response time limitations which in turn limit the maximum coding rate. Moreover, latency within the critical feedback path which must be accounted for has proven difficult to control during manufacturing. Alternative designs (for example a T flip-flop driven by a clock gated by the input data stream) have similar timing constraints that limit coding rate and make manufacturing more difficult.
Differential Phase Shift Keying (DPSK) systems use differential encoders to encode baseband signals for phase modulation on a carrier wave. A modulator shifts the carrier phase in discrete increments producing symbols corresponding to the bit pattern in the encoded baseband signal. A transmitter transmits the modulated carrier to a receiver over a communication channel. At the receiver, a phase comparator detects changes in phase of the carrier recovering the transmitted symbols. The symbols represent the bits of the encoded baseband signal. A differential decoder decodes the encoded baseband signal producing the original data stream bit pattern.
In an optical DPSK system a Mach-Zehnder modulator generates laser symbols from a differentially encoded bit pattern. The laser light travels over a communication channel and is detected by a delay interferometer. The delay interferometer sums the received light with light received one bit earlier, forming a light signal whose presence or absence indicates the presence or absence of a carrier phase change between the two bits. A photodiode converts the light intensity into an electrical signal. A circuit processes the electrical signal to produce the original bit stream.
The carrier signal in an optical system inherently has a large frequency and therefore accommodates a very large signal bandwidth or data rate. The data rates achievable in optical systems with DPSK modulation are often limited more by the data rate capability of the differential encoder than by other circuits or the channel (fiber or free space). Conventional differential encoders can encode at rates up to about 15 GHz. At frequencies higher than 15 GHz, feedback latency or other timing problems cause rapidly increasing design difficulties which may result in unacceptable encoding errors.
Those concerned with the development of high data rate communication systems have long recognized the need for faster and more accurate differential encoders. The present invention significantly advances the prior art by providing a high speed differential encoder that can be used to achieve higher data rates in optical DPSK systems.